Technology update

Jul 14, 2014

Joining up the dots

By combining carbon dots that emit blue light and zinc copper indium sulphide quantum dots that emit in the green and red regions of the electromagnetic spectrum, researchers in China and the US have succeeded in making white light-emitting diodes with a high colour-rendering index of 93. The devices might find use in solid-state lighting and displays.

Lighting consumes around 20% of all globally generated electrical energy. Light-emitting diodes (LEDs), and especially white LEDs (WLEDs), could help reduce this figure.

At present, most WLEDs contain the YAG:Ce yellow phosphor, but this type of LED has a low colour-rendering index (CRI) because it only weakly emits in the red spectral region. Replacing YAG:Ce with semiconducting quantum dots (QDs) might be one way to overcome this problem, if it were not for the fact that most high quality QDs contain cadmium or lead, which are highly toxic.

Researchers have recently suggested using carbon dots (CDs), not only as more environmentally friendly alternatives, but also because they absorb light over a broad range of light wavelengths. However, there is another problem here in that these materials generate rather “cold” light, which is not very comforting. This is because they do not emit much light in the yellow and red regions.

Making cold light warmer

Although scientists have already overcome this drawback by combining these blue-emitting CDs with multicoloured QD phosphors to increase the CRI, such phosphors are still based on cadmium. So the problem has annoyingly come full circle.

A team led by Andrey Rogach of City University in Hong Kong and William Yu and Yu Zhang of Jilin University is now saying that it has found a way around this issue once and for all. The researchers have fabricated WLEDs by combining carbon dots that emit blue light with yellow- and red-emitting Cd-free zinc copper indium sulphide core/shell QDs. The devices, which were made by mixing suitable quantities of the two types of material with the polymer PMMA on the surface of a blue LED chip, have a very high CRI of 93. This value is higher than the CRI (of less than 75) of the best YAG:Ce commercial WLED.

Emitting over a broad range of light wavelengths

“The high CRI in our WLEDs comes thanks to the fact that the ZnCuInS QDs emit over a broad range of light wavelengths,” explains Zhang. “The devices show promise for use in solid-state lighting and displays.”

The team, which includes researchers from Louisiana State University, says that it is now focusing on the structure and surface of its devices to further improve their efficiency. “We are also trying to make the LEDs emit different colours by tuning the nanoparticle size, the thickness and number of emitting layers” Zhang tells nanotechweb.org. “Some of our recent work has already been published in Appl. Phys. Lett., ACS Nano and Phys. Chem. Lett., for example.”